596 PHYSIOLOGY. 



root and stem to a progressive downward growth : thus the new cells 

 were supposed to be formed from above downwards. It is not necessary 

 to state the argument on which this theory was based, as it has been com- 

 pletely set aside by the researches of Trecul and others, which show that 

 the new tissues are formed at the spot where they are seen, and are not 

 formed from above downwards. Both bark and wood cooperate in the 

 formation of new wood ; and either of them may form woody or cortical 

 tissues without the intervention of the other. 



There is reason to believe that the growth of the stem of trees takes 

 place principally in the summer months, often in a few weeks, and that 

 comparatively little increase takes place either in spring or in autumn, 

 though in the latter period the new growths are consolidated. 



Growing-points of Cryptogamia. Cryptogams frequently, but 

 not universally, differ from Phanerogams, in addition to other 

 matters elsewhere referred to, in the circumstance that their grow- 

 ing-point terminates in a single apical cell (fig. 600), from the 

 repeated subdivision of which in various directions the stems, roots, 

 and their subdivisions originate. No such solitary apical cell is 

 met with in Phanerogams, but a group of cells, as just alluded to. 



Sect. 4. DEVELOPMENT AND GROWTH OP THE ROOT. 



The general structure of the roots has been previously men- 

 tioned (p. 534). It is, however, requisite in this place to allude 

 to the course of development as seen at the growing-points, which 

 has been made the subject of investigation by JN T ageli, Leitgeb, 

 Haustein, Reinke, and others, especially Janczewski. 



In Vascular Cryptogams the growing-point of the root is marked by a 

 single apical cell, which undergoes division in two different directions, 

 obliquely and transversely. The new cells formed by oblique segmenta- 

 tion go to make up the body of the root, while the cells formed by trans- 

 verse division go to constitute the root-cap. The terminal cell of the root 

 (as of the stem) in Lycopods varies in shape ; sometimes it is a three- 

 sided pyramid, sometimes a four-sided pyramid, sometimes it is lenticular. 

 The direction of ramification of the roots of these plants (also the phyllo- 

 taxis of the leaves in the case of the stem) is intimately associated with 

 the form and mode of subdivision of the apical cell. The elongation of 

 the body of the root is described by Nageli and Leitgeb as due to succes- 

 sive divisions of the terminal cell in a spiral direction, the direction of the 

 spiral being usually dextrorse and homodromous. 



In Phanerogams there is no single apical cell, but a group of cells of 

 the same relative age and order. Janczewski arranges roots, considered 

 with reference to the growth of their growing-point, under five heads : 

 1. The growing-point is made up of four primarily independent tissues, 

 the root-cap, the epiderm, the cortex, and the central cylinder : the roots 

 of Hydrocharis Morsm Ranee (Frogbit) and of Pistia Stratiotes belong 

 to this type. 2. The growing-point is destitute of epiderm, the outer 

 layer of the cortex answering the purpose : the roots of the Onion and 

 of many other Monocotyledons afford examples of this type. 3. The 



